In Matras, monsters seem to be unable to enter the city. I tried duplicating the same conditions in my world, but they're still able to enter. I'm aware that I could add doors/gates that teleport the player in and out of my cities, but I'm not very keen on that idea.
Also, are there any workarounds to prevent monsters from following you 20+ squares away? Are there any objects a player can pass through that a monster cannot?